The present invention is related to an apparatus and a method for the representation of a geometrical figure on the surface of a patient's body that is situated on a support, and relates to the field of radiation therapy with ionising radiation for treating cancer. In this, usually plural rays from different directions are directed towards the body to be treated, so that they intersect in an isocentre. Here acts the summed-up radiation dose of the different ionising rays directed to the isocentre. In this way, the adverse effect to the surrounding tissue is minimised.
A computer tomography (CT) mapping of the patient positioned and fixed on a support, like a treatment table, is usually established as the first step of a radiation therapy. Based on this mapping, a 3D-model of the patient is made and the tumour to be treated is localised and an irradiation plan is set up. This comprises contouring the target volumes as well as the calculation of the dose for the irradiation, and in this the determination of the number and location of the radiation areas of the irradiation machine in particular, so that the tumour is irradiated as desired.
After the mapping of the patient, the patient is no more on the support. Furthermore, a plurality of irradiation dates (usually up to 30 fractions) takes usually place in the frame of a subsequent radiation therapy. Therefore, a transfer of the radiation areas calculated on the basis of the established 3D-model of the patient to the real patient is repeatedly necessary. This step is also called “simulation”. In particular, the patient must be reproducibly laid on the treatment table such as he/she lay when the CT mapping was established.
In order to do this, it is known to perforin a positioning of the patient by means of 3D radiography (“Cone Beam”), wherein bones of the patient constitute the reference points for an alignment of the patient, for instance. The advantage of this method is a high accuracy. In addition, no reference marks or the like have to be affixed on the patient's skin. The disadvantage of this method is a high x-ray radiation dose for the patient, because the x-ray positioning is required before each treatment fraction. That is to say, in the frame of a radiation therapy, this may usually take place up to 30 times. This radiation dose applied to the body in a great volume can induce a new cancer again in the long term, in younger patients in particular.
According to a known alternative method, the patient can be positioned on the support by means of three reference points on its skin, which are formed by suitable retro reflectors, for instance. Such a method is known from DE 44 18 216 A1, the entire contents of which is incorporated herein by reference. After performing the irradiation planning, the intersection points of the irradiation areas with the surface of the skin which are necessary for the desired irradiation are calculated. The intersection points are then at hand in the form of a 3D coordinate table. After newly positioning the patient on the basis of the three reference marks, the coordinates from the table can then be represented on the body surface of the patient one after the other with a laser system. The approach to the coordinates can be controlled by an operator, for instance by an infrared remote control. The respective points represented by the laser are then manually drawn in on the skin of the patient, with a pencil for instance. The desired intersection area for the irradiation results by connecting the points.
Such a method is known from DE 44 21 315 A1 or DE 195 24 951 A1 the entire contents of which is incorporated herein by reference. The laser device used for this consists of five motor movable lasers in particular, wherein two lasers adjustable in the height direct a horizontal line along the treatment table to the patient, at the right and at the left side from the treatment table, respectively. The remaining three lasers are mounted in a plate which is situated above the treatment table, a CT table for instance. In this, one laser is movable transversely to the table's long side direction and directs a line along the table to the patient. The two remaining lasers in the plate are coupled with each other and direct a common line to the patient, transversely to the table's alongside. By the coupling of two lasers, even such coordinates can be represented on the skin of the patient which otherwise would be shaded below the transverse diameter of the patient. With the described system, it is possible to approach almost arbitrary coordinates on the skin of the patient, wherein one coordinate is always indicated by a cross of two laser lines. Such a laser system is offered by the applicant under the name “Dorado CT4”.
However, the described method is relatively time-consuming and therefore expensive, because usage time in the CT-room is very expensive. In addition to this, manual drawing in the points is not always sufficiently accurate, depending on the adiposity of the patient in particular. In particular, skin marks can shift. Therefore, marks of the intersection areas on the skin are performed only partially in practice. Correspondingly, the correct irradiation of the patient is not always ensured in a sufficiently accurate way.
Moreover, there is a problem in that the position of the patient between the irradiation planning and in particular in a mapping of the patient's body performed in this on the one hand, and a later irradiation date on the other hand, is inevitably not always identical, in spite of the utilisation of identical positioning aids on the treatment table. The procedures for reproducible positioning of the patient explained above are relatively sumptuous and do not always achieve the necessary accuracy in practice.
The simulation is not only an assignment of the irradiation plan to the patient, but also an important element in the context of the quality assurance. The irradiation plan is checked for its plausibility for the last time before the therapy and it is determined whether the ray areas can be reproducibly adjusted to the desired planned target volume.
Based on the clarified state of the art, the present invention is based on the objective to provide an apparatus and a method of the kind indicated in the beginning, by which a reproducible, accurate positioning of the patient is possible in a reliable manner.